The present invention relates to a device for humidifying air and to a method for air humidification using this device.
The use of air with a defined humidity is desirable or required in many cases, for example in air-conditioning technology or in medicine. Air from compressed-air lines or cylinders is generally too dry, while atmospheric air exhibits considerable fluctuations in terms of its humidity and, moreover, its composition often fails to satisfy requirements.
Various methods are used to humidify air. In most cases, water is sprayed or vaporized. For example, a conventional method uses, for example, an open water reservoir, from which liquid is pumped out and sprayed into the flow of air. The water which is not taken up by the flow of air flows back into the reservoir. The microorganisms introduced multiply in this water reservoir and can only be controlled by the use of chemicals. Therefore, in many situations the humidified air is undesirably polluted with chemicals. To counteract the microorganisms, methods have been developed according to which the humidification water was heated. However, it has emerged that even dead microorganisms or their cell constituents may impair the respiratory tract of humans. Therefore, alternative methods have been developed which envisage water vapor being introduced into the air to be humidified. In these methods, the water is heated to boiling point and only the vapor phase is used for the humidification. A drawback of these methods is the long mixing paths required in order to achieve uniform humidification. Finally, it is known for water to be sprayed directly into the room which is to be air-conditioned. This, however, requires an optimum distribution of a very large number of nozzles in order to achieve uniform humidity throughout the room. Moreover, this system cannot be integrated into an existing air-conditioning unit without further measures.
It is known, for example from EP 0 370 540 A1 and U.S. Pat. No. 5,348,691, to sterilize water by filtration using membranes. According to the methods described in those documents, dry air from a blower is passed through a membrane contactor, the membrane contactor separating the liquid phase, i.e. water, from the air and exposing the airflow to a large area of water in a very small space. The water molecules evaporate from the water surface and pass into the airflow. The membranes employed are hydrophilic and hydrophobic membranes, preferably in capillary form. For the generally large airstreams, it is necessary to provide a large membrane area combined with a low loss of flow pressure. With both flat and hollow fiber membranes, this leads to very short modules with very large end faces, i.e. inlet surfaces. To achieve the required low pressure losses, the flow channels have to be relatively large or the hollow fiber diameters must not be too small. However, this entails the drawback that it is impossible to achieve a high packing density. In addition, large-scale production of modules of this nature has not yet been tried and will certainly entail a very complex method.
The present invention is therefore based on the technnical problem of providing an air humidifier which overcomes the abovementioned drawbacks, in particular provides a large contact area between water and air combined, at the same time, with low losses of flow pressure.
The present invention solves this technical problem by providing an air humidifier, comprising a membrane contactor, in which the membrane contactor has at least one tube membrane embedded in a hydrophilic, porous body. In a particularly preferred embodiment of the invention, the hydrophilic, porous body is a body made from a ceramic material, referred to below as a ceramic body. The body may consist exclusively of the ceramic material or may contain substantial amounts, preferably more than 50% by weight, based on the weight of the body, of this material. According to the invention, it is possible, for example, to use coarse ceramic filters such as are used for filtering molten metals. In another preferred embodiment of the invention, the hydrophilic, porous body is made from a polymer, i.e. consists of this polymer or contains substantial amounts, preferably an amount of more than 50% by weight, based on the weight of the body, of this polymer. Naturally, it is also possible to use hydrophilic, porous bodies made from other materials, provided that these materials are able to ensure that the water is dispersed over a large surface area. According to the invention, organic or inorganic woven fabrics or woven-fabric-like structures consisting of or containing substantial proportions, preferably more than 50% by weight (based on the weight of the body), of porous filament material, may also be used as the only or principal constituent of the body. The body should be hydrophilic and have a porous structure, i.e. a structure of increased surface area. In addition, it is necessary for the hydrophilic, porous body to be sufficiently strong and rigid for the incoming flow of air not to cause any compression and therefore any significant change in the pore structure and flow velocities. Therefore, bodies which are used according to the invention preferably have a self-supporting, non-compressible structure.
In a preferred embodiment of the present invention, the diameter of the pores in the hydrophilic, porous body is less than or equal to 5 mm.
In connection with the present invention, a membrane contactor is understood as meaning the unit according to the invention comprising at least one tube membrane and hydrophilic, porous body. The function of a membrane contactor is to sterilize the water by means of the at least one tube membrane contained therein, while the hydrophilic, porous body serves as an area-enlarging contact means between air and sterile water. The membrane contactor according to the invention may be fitted, in conventional air humidifiers, in the form of a module individually or in series or parallel with other modules and the water supply. The use of the membrane contactor according to the present invention in an air humidifier is possible wherever humidified, sterile air is required, for example in air-conditioning units, be they stationary, for example in building, or in mobile units, such as vehicles, in room air humidifiers, sterile working areas, operating theaters, laboratories, for the production of medicaments or sterile appliances or for storing valuable cultural materials, such as books, or in storage and exhibition areas, museums, etc.
In the context of the present invention, a membrane is understood as meaning a technical membrane, particularly preferably a thin, film-like, microporous separating layer. A porous membrane of this nature may have a foam-like mesh structure. According to the invention, it is equally possible to use homogenous membranes and membranes of asymmetric structure, for example comprising a supporting layer and a separating layer.
The membrane according to the invention is therefore preferably a membrane filter which particularly preferably may be produced, for example, from ceramic or polymeric material, for example cellulose derivatives, polyamides, polyvinyl chloride, polysulfone and/or teflon and consists of this material or contains substantial proportions of this material, in particular more than 50% by weight. The membranes are preferably 50 to 250 xcexcm thick.
The membranes according to the invention are designed as tube membranes.
According to the invention, it is also possible to combine capillary membranes or hollow fiber membranes to form bundles of flexible tubular bodies and to integrate these bundles in the hydrophilic body.
In a particularly preferred embodiment, the invention provides for the at least one tube membrane to be a ceramic tube membrane or a polymeric filtration membrane, in particular a microfiltration membrane. Naturally, it is also possible to use other tube membranes, provided that they are of substantially tubular form and have a pore diameter which allows microorganisms, for example bacteria, viruses, cells of human, animal or plant origin, parts thereof and/or high-molecular substances to be blocked. It is particularly preferable for the tube membranes to have pores with a diameter of  less than 0.2 xcexcm, in particular from 0.01 to 0.20 xcexcm. It is also possible to use hollow filtration fibers, in particular hollow microfiltration fibers.
In a preferred embodiment of the present invention, the internal diameter of the tube membrane lies in a range from 200 xcexcm to 16 mm.
The invention therefore provides for the water, which may contain microbes, to be passed through at least-one tube membrane, that is to say a membrane which is of tubular or hose-like form, into a hydrophilic, porous body. In the hydrophilic, porous body the water passes through the pores of the membrane into the body. According to the invention, it is now envisaged that the pores of the membrane will be sufficiently small for it to be impossible for either microorganisms or fragments of lysed microorganisms or larger molecules, which may have a toxic or allergenic action, to pass through the membrane. The tube membrane therefore acts, as it were, as a filter for undesirable constituents of the water. The water leaving the tube membrane is therefore sterile. This water passes into the pores of the porous body, where it is dispersed by capillary forces over the entire inner and upper area of the body. The air which is to be humidified is passed through the porous body, where it can take up sterile water over a relative large surface area.
The actual membrane area, that is to say the area of the tube membrane, may advantageously be relatively small, since the exchange area is for the most part provided by the inner and outer surfaces of the hydrophilic porous body and the filtration capacity of the tube membranes used according to the invention is very high, reaching levels of more than 1000 l/M2 h bar.
In a particularly preferred embodiment, the invention provides for the tube membrane to pass in meandering or worm-like form through the porous body. Naturally, it is also possible for a plurality of tube membranes to pass through the body, for example in parallel, although they may also pass through the body in any other desired way. It is preferable for the at least one tube membrane to pass through the hydrophilic, porous body in such a way that a uniform supply of water to the entire region of the hydrophilic, porous body which is reached by the flow of air is ensured, in which case the distribution of the tube membranes has to be adapted to the size and porosity of the hydrophilic body and to the flow of air.
In another preferred embodiment, the invention provides for the hydrophilic, porous body to be cuboidal and, accordingly, to have four side faces and two inlet and outlet faces which lie opposite one another. In a further preferred embodiment of the invention, both the inlet surface area and the outlet surface area of the cuboid are larger than each of its individual side faces, preferably by a factor of from 2 to 50, particularly preferably 10 to 30.
It is also possible for the areas of the inlet and outlet surfaces to differ, for example for the outlet surface area to be larger than the inlet surface area, in order in this way to enhance the fluid dynamics.
In another preferred embodiment of the invention, the ratio of the inlet surface area of the hydrophilic, porous body to the area of the tube membrane incorporated in the hydrophilic, porous body is from 10 to 25, preferably 15 to 25, in particular 20.
In a further preferred embodiment, the invention envisages the longitudinal axis, preferably the longitudinal axes, of the at least one tube membrane, preferably of the plurality of tube membranes, to lie in at least one plane which is parallel to the inlet surface of the hydrophilic, porous body. In this case, it is possible for the tube membrane to lie in one or more planes parallel to the inlet surface of the hydrophilic, porous body.
The invention also provides an air humidifier which contains a housing and air-supply and air-discharge devices, a membrane contactor according to the invention and, if appropriate, devices for moving the air, such as pumps or fans. The invention also envisages that the membrane contactors of the present invention can be fitted together to form a modular system comprising a plurality of membrane contactors, in which case it is entirely possible for a plurality of hydrophilic, porous bodies to use one or more common tube membranes, i.e. this/these tube membrane(s) run(s) through a plurality of bodies. Naturally, the invention also relates to air-conditioning systems which contain membrane contactors or air humidifiers in accordance with the invention.
The invention also relates to a method for the sterile humidification of air, in which water is passed into the at least one tube membrane of an air humidifier of the present invention, from where it passes through the pores of the tube membrane into the hydrophilic, porous body of the air humidifier or membrane contactor, where it is dispersed and taken up by a flow of air which is being passed through the hydrophilic, porous body.
In a further preferred embodiment, the invention provides a method of the abovementioned type in which the flow of air is introduced into the hydrophilic, porous body of the membrane contactor perpendicular to the longitudinal axis of the tube membrane.
In this case, the invention has the advantage that there are short diffusion paths for the water perpendicular to the direction of the flow of air and that the short paths, the form of which differs considerably in flow cross section, of the pores through which the medium flows, which pores also have cross-connections with other pores, means that laminar flows are not formed. According to the invention, this has the advantage that there is no need for a mixing zone downstream of the humidification zone.